What term refers to a massive collection of stars, star systems, and gas bound together by gravity?

A galaxy is defined as a massive collection of stars, star systems, gas, dust, and dark matter that are bound together by gravity. Galaxies can vary significantly in size and structure, containing anywhere from millions to trillions of stars. The gravitational forces hold these components together, allowing them to form complex structures, including spiral arms, elliptical shapes, or irregular patterns.

In contrast, an open cluster refers to a group of a few hundred to a few thousand stars that are loosely bound by gravitational attraction and are typically young. A binary star system consists of two stars orbiting around a common center of mass, which doesn't encompass the vast collections typically represented by galaxies. Quasars are extremely bright and distant objects powered by black holes at the centers of some galaxies, but they are not a collection of stars themselves.

Therefore, the term that best describes a massive collection of these components, which includes the organizing forces of gravity, is indeed a galaxy.